# Why does oxygen (O) not have a subscript/number?

Elemental oxygen is a diatomic molecule, and we represent it as ${O}_{2}$, or as dioxygen. This is a different species from atomic oxygen, $O$, and we have to put in energy to make the transition to this species:
${O}_{2} \left(g\right) + \Delta \rightarrow 2 O \left(g\right)$